Zinc die casting steels



May 20, 1958 v. KLAYBOR ZINC DIE CASTING STEELS Filed July 2, 1957INVENTOR Leonard V Klaybor ZINC DIE CASTING STEELS Leonard V. Klaybor,Dunkirk, N. Y., assignor to Allegheny Ludlum Steel Corporation,Breckenridge, Pa., a corporation of Pennsylvania Application July 2,1957, Serial No. 673,550 Claims. (Cl. 75-124) This invention relates tosteel and in particular to a die casting steel suitable for use in thedie casting of molten zinc.

Heretofore there have been many steels that have been made and used asdies in the die casting of zinc. While special emphasis has been placedon abrasion and thermal shock characteristics in these steels, afundamental problem has existed for some time in using the known diecasting steels for such applications. In the past, where the knownsteels have been used in contact with molten zinc, the molten zincappears to attack the material of the die combining with the ironthereof to form a coating of a zinc-iron compound. This zinc-ironintermetallic compound is extremely brittle and when formed on thesurface of the die causes portions of this coating to flake off removinga portion of the die surface thereby producing premature die failure.This difiiculty is most apparent when the steel is used as a sprue fordie casting zine, an application of the die steel where the hottestmaterial under the highest pressure is applied to the die and where theworst die life is usually obtained. In such application, the known dieshave a maximum life as measured by the number of shots of molten zincpassing through the sprue of only about 100,000 shots.

An object of this invention is to provide a steel for use in die castingmolten zinc and alloys thereof.

A specific object of this invention is to provide a steel suitable foruse in die casting molten zinc and alloys thereof and comprising fromabout 0.25% to about 0.75% carbon, from about 1.10% to about 5.00%manganese, from about 0.10% to about 1.00% silicon, from about 0.10% toabout 1.00% chromium, from about 2.0% to about 4.0% aluminum and thebalance iron with incidental impurities, said steel being characterizedby its resistance to formation of brittle intermetallic zincironcompound in the presence of molten zinc.

These and other objects of this invention Will-become apparent to oneskilled in the art when taken in conjunction with the followingdescription and the drawing, the single figure of which is aphotomiorograph taken at a magnification of 500 times of a die castingsteel embodying the teachings of this invention.

In its broader aspects the alloy of this invention comprises about 0.25%to 0.75% carbon, about 1.10% to about 5.00% manganese, about 0.10% toabout 1.00% silicon, about 0.10% to about 1.00% chromium, about 2.0% toabout 4.0% aluminum, and the balance substantially all iron withincidental impurities, each of the alloying elements performing aspecific function in the steel. The carbon, manganese and chromium arethe principal hardening agents within this alloy, it being found that aminimum of 0.25 carbon is required in order to obtain a hardness of 300Brinell or greater as will be ref-e red to hereinafter. Manganese andchromium are the principal elements for providing the hardenabilityrequired in the alloy of this invention. The manganese content isextremely critical in the alloy of this invention in that at least 1.10%manganese is necessary in order to obtain the minimum hardenabilitybased on an oil quenching medium used to quench the alloy from the heattreatment temperatures which will be more fully described hereinafter.The aluminum is believed to function in a manner also to be describedhereinafter.

The alloy of this invention may be made in any of the well-knownmanners, for example, by electric furnace arc melting. In making thesteel, predetermined quantities of scrap and/or hot metal are placed inan electric furnace together with sufiicient alloying elements andfluxing agents in order to produce the steel of desired analysis. Thispractice is common in the art and will not be described in detail. Themolten steel of the prescribed chemical composition is cast into ingotswhich, when solidified, are then forged at a temperature in the rangebetween 1900 F. and 2100" F. While forging is preferred, it will beappreciated that hot pressing or any other operation usually used in thefabrication of steel from ingot to a semi-finished mill product, forexample; billet or bar or other similar shape, may be used. The forgingis followed by an annealing treatment at a temperature of about 1500 F.from which temperature the. semi-finished mill product is permitted tocool slowly.

Thereafter the annealed material is hardened by heating at a temperaturein the range between 1800 F. and 1900 F. for a time period rangingbetween one-half hour and four hours, and then quenched in air or oil,depending upon the size and composition as will be described more fullyhereinafter, and tempered at a temperature between 900 F. and 1200" F.for a time period ranging between 3 and 10 hours. This results in asteel having a hardness between 300 and 375 Brinell. The steel may thenbe machined to the shape and size of the die, still retaining thehardness characteristics.

Referring to Table I, the composition of the steel of this invention isset forth in the form of the general range, the optimum (1) range for anair quenching steel, the optimum (2) range for an oil quenching steeltogether with a preferred air hardening composition as alloy A.

Table I General Optimum Optimum AlloyA Range 0.a0-0.50 0.37 1. 10-3.25 I3.09 0. 20-030 0.27 0.40-0.50 0.45 2. -3. 25 3. 05 Bal. B81.

From Table I it can be seen that the manganese content determines thequenching medium of this steel. It has been found that a manganesecontent between, 2.75% and 5.00% is needed in order to impart thecharacteristic to the steel of this invention of attaining a desiredhardness and microstructure by air quenching. On the other hand, if anoil quenching medium is to be employed, then the manganese content ofthe steel may be reduced to as little as 1.10% without impairing theresulting hardness and microstructure.

In order to more clearly illustrate the highly critical nature of themanganese content, reference is directed to Table II which lists thechemical composition of a series of alloys having a varying manganesecontent. It

is to be noted in Table II that the composition of the.

Since the effect of the manganese is highly critical from the standpointof hardenability, the hardenability characteristic of the subject alloywas measured using the standard Jominy end quench test. The criterionused to determine the acceptable hardenability characteristic of thealloy was a hardness of at least 32 R at a distance of twelve sixteenthsfrom the quenched end. The alloys were preheated to 1500 F. andthereafter the tempera ture was raised to 1800 F. and held for 10minutes at that temperature. The alloys were then transferred to theIominy test stand and quenched according to the standard practice.

Referenceis directed to Table 111 which illustrates the efifect ofmanganese on the hardenability of the alloys of Table II.

to such an extent that the alloys have a minimum hardness penetration of32 R at a depth of thirty and twentyfour sixteenths, respectively.

Substantially similar results were obtained in alloys Nos. 6, 7, 8, 9and 10 which are nominally a 2% aluminum alloy. Thus the data for alloysNos. 6 and 7 having a manganese content of 0.53% and 0.71% show that thealloys have a shallow hardenability. However, for alloys Nos. 8, 9 and10, containing 1.12%, 1.26% and 1.6% manganese, respectively, it isclear that these alloys possess sufiicient hardenability to be used inthe dies for die casting zinc alloys.

It is significant to point out that alloys of this invention arehardenable to such an extent as to possess a hardness of at least 300BHN. Since the hardening takes place by means of a phase transformationphenomena of austenite to martensite, the data of Table III clearlysubstantiating this, it follows that the addition of about a minimum of2% aluminum, a strong ferrite forming element, does not affect theability of this alloy to form austenite upon heat treatment. Heretofore,it was believed that the addition of 2% or more of aluminum to an alloycontaining up to 04% carbon, up to 0.8% silicon, up to 0.8% chromium, upto 0.8% manganese and the balance iron would completely suppress thegamma loop and eliminate the austenite phase thus making the alloyincapable of hardening through phase transformation. The data containedin Table III clearly refutes such contention.

Table III [Hardness (B0)] Alloy No 1 2 3 4 5 6 7 8 9 10 Mn Content 0.450.76 1.05 1.57 1.5 0.53 0.71 1.12 1.26 1.60

Distance from Quenehed End in 1612115 of an Inch:

1 47 37 43 44 2 51 48.5 50.5 53 51 34 34 43 51 53 49 53 53. 5 53. 5 53.5 29 41 51 51 42 53. 5 54 53. 5 53. 5 23 25. 5 3s 4s 4s 50 52 54 54 2624. 5 46. 5 46 46 29 54 53 24. 5 23 35 45 45 26 40 4s 52. 5 53 22. 5 134 43. 5 42 25 36 44. 5 50 53 21 20 5 33.5 41 41 23. 5 33 42.5 47 52 2019 31. 5 40. 5 40. 5 23 31 40. 5 44 51. 5 19 17. 5 31 41 4o 22 20 39. 541 49 13 16. 5 29. 5 40 39. 5 22 27 37 40 47 17 17 23. 5 39. 5 3s. 5 21.5 25. 5 34. 5 38 45 18 16.5 28 39 38 21 25. 5 32 36. 5 44 17. 5 15. 527. 5 3s. 5 26. 5 20. 5 23. 5 33 34. 5 44 17. 5 16 26 37. 5 37 19 25 31.5 33 42 17 16 25.5 37.5 37 19 24 31 31. 5 41 16 16 27 36 35 18. 5 21.529' 30 39 16 16 26 35. 5 34 17 21 28 29 38 16 15. 5 26 35 33 16 19 27 2936. 5 v16 15 23 34. 5 32 15. 5 1s 26 30 35. 5 16 15 19. 5 33 29. 5 1519. 5 24. 5 31 35. 5 16. 5 15 1s. 5 32. 5 2s 15 19. 5 24 2s 35 16 14. 519 32 2s 15 1s. 5 24 26. 5 32. 5 16. 5 14. 5 20 32 26. 5 13. 5 18 19. 526 32. 5 16.5 14.5 21 30 25 13.5 18 21 25 32 14. 5 14 2o 23 24. 5 14 1713 23. 5 32 15. 5 14 20 27. 5 23. 5 13. 5 15 18. 5 24 32 14. 5 14 21 2723. 5 14 16 18 23 31 15 14 21 27 24 15 17 19.5 21.5 29. 5 15 14 21. 526. 5 24 14. 5 15 21 24 28 17 15 2o 27 25 14. 5 18 24 25.5 27 15.5 14 202s 25. 5 15 13 25 2 26 From the data contalned 1n Table III 1t 1s clearthat Examinanon of steel dies having a composition within manganeseexerts a pronounced effect on the alloy of this invention. By comparingthe test results for alloys Nos.'1, 2 and 3, it is seen that increasingthe manganese content from 0.45% to 1.05% in a nominal compositioncontaining about 3% aluminum is eifective for increasing hardenability.However, even with 1.05% manganese present, the alloy does not possesssuflicient hardenability to obtain a hardness of 32 R (297 BHN) at adistance of twelve sixteenths from the quenched end. Since this is thecriterion of acceptability for the alloy of this invention it followsthat the manganese content must exceed 1.05%. The data for alloys Nos. 4and 5 having a manganese content of 1.57% and 1.5%, respectively,clearly illustrate that a manganese content of about 1.5% is sufficientfor increasing the hardenability the general range given hereinbeforereveals that the aluminum content in the steel cooperates with thecarbon content in the steel to form very small islands of ferrite in theresulting steel. These islands or" ferrite are evident in the figure,which its a photornicrograph taken at a magnification of 500 times ofthe die casting steel having thecomposition of alloy A as given in TableI which was forged, annealed at 1500 R, and thereafter slowly cooled,hardened at temperature of 1850 F. and thereafter tempered at atemperature of 1100 C. In the photomicrograph the structure of the steelis seen to comprise a matrix of tempered martensite 10 having islands offerrite 12 contained therein. It is believed that these small islands offerrite 12 function to improve the machineability of the heat treated,hardened composi- I tion of the steels of this invention withoutproducing any adverse effect in their use as die steels.

In practice, it is preferred to increase the aluminum contentproportionally within the broad range given as the carbon content isincreased within the given broad range to thereby insure the presence ofthe small islands of ferrite 12 in the matrix of tempered martensite 10.While the composition of the steel is varied within the broad rangegiven in Table I hereinbefore, it is preferred that in the optimum rangethe aluminum content be increased from 2.75% to 3.25% in directproportion to and as the carbon content increases from 0.30% to 0.50%.When such relationship is maintained between the aluminum and carboncontents, it is found that the very small islands of ferrite 12 arealways present as illustrated in the figure and that the resulting steelhas excellent machineability.

In use it is believed that the aluminum content of the steel forms avery thin transparent microscopic protective film of aluminum oxide onthe surface of the die which functions to protect the die from theformation of the brittle intermetallic zinc-iron compound which isformed on the known die steels of the prior art when employed in zincdie casting applications. This thin protective coating of aluminum oxideis adherent to the surface of the die and clearly increases theoperating life of the die.

As a specific example of the exceptionally long die life found with diesformed from the steel of this invention, reference may be had to thespecific composition of alloy A given in Table I. When such steel isformed into a die and used as the sprue in the die casting of zinc, ithas been found that 280,000 shots of molten zinc have been passedtherethrough without any sign of die failure. Such die life isexceptional when compared to the die life of the prior art die steelsused in such applications, and it is believed that the presence of thealuminum in the alloy of this invention makes possible such an in-vcrease in the die life. When dies formed of the steel describedhereinbefore are examined after use as the sprue for die casting zinc,it is found that such dies are entirely free of the brittleintermetallic zinc-iron coatings found on the surfaces of the prior artdies after such use.

The steel of this invention requires no special skills or apparatus inits preparation and use. The heat treatments are relatively simple andthe alloy composition can be readily reproduced by anyone skilled in theart.

Further, it is to be noted that the alloying content of the steel is lowand there is a significant absence of strategic alloying elements.

This application is a continuation-in-part of application Serial No.499,445, filed April 5, 1955, now abandoned.

I claim:

1. A die steel for use in die casting molten zinc and alloys thereofconsisting of, from about 0.25% to about 0.75% carbon, from about 1.10%to about 5.0% manganese, from about 0.10% to about 1.00% silicon, fromabout 0.10% to about 1.00% chromium, from about 2.0% to about 4.0%aluminum, and the balance substantially all iron with incidentalimpurities.

2. A die steel for use in die casting molten zinc and alloys thereof,consisting of, about 0.30% to 0.50% carbon, about 2.75% to 3.25%manganese, about 0.20% to 0.30% silicon, about 0.40% to 0.50% chromium,about 2.75 to 3.25 aluminum, and the balance iron with incidentalimpurities, said steel being characterized by being hardened in air to ahardness of at least 300 Brinell Hardness Number.

3. A die steel for use in die casting molten zinc and alloys thereof,consisting of, about 0.30% to 0.50% carbon, about 1.10% to 3.25manganese, about 0.20% to 0.30% silicon, about 0.40% to 0.50% chromium,about 2.75% to 3.25% aluminum, and the balance iron with incidentalimpurities, said steel being characterized by being hardened byquenching in oil to a hardness of at least 300 Brinell Hardness Number.

4. A die steel for use in die casting molten zinc and alloys thereof,consisting of, about 0.37% carbon, about 3.09% manganese, about 0.27%silicon, about 0.45% chromium, about 3.05% aluminum, and the balanceiron with incidental impurities.

5. A die steel for use in die casting molten zinc and alloys thereof,consisting of, from 0.30% to 0.50% carbon, about 1.10% to 3.25%manganese, about 0.10% to 1.00% silicon, about 0.10% to 1.00% chromium,about 2.75% to 3.25% aluminum, and the balance iron with incidentalimpurities, said carbon content varying in direct proportion withrespect to a variation in the aluminum content with the ranges given.

References Cited in the file of this patent FOREIGN PATENTS 304,303Great Britain May 21, 1930

1. A DIE STEEL FOR USE IN DIE CASTING MOLTEN ZINC AND ALLOYS THEREOFCONSISTING OF, FROM ABOUT 0.25% TO MANGANESE, FROM ABOUT 0.10% TO ABOUT1.00% SILICON, FROM ABOUT 0.10% TO ABOUT 1.00% CHROMIUM, FROM ABOUT 2.0%TO ABOUT 4./% ALUMINUM, AND THE BALANCE SUBSTANTIALLY ALL IRON WITHINCIDENTAL IMPURTITIES.